Cryogenic particle blasting is now relatively well known. The process generally utilizes sublimable particles, such as carbon dioxide pellets, which are propelled against a work piece for a variety of reasons, such as, for example, to remove contaminates or coatings from the surface of the work piece Although carbon dioxide is referred to herein, it will be understood that any cryogenic sublimable material may be used.
The effectiveness and efficiency of the cryogenic blasting process depends at least in part upon the type of contaminate or coating being removed and the nature of the surface from which it is being removed. Problems which typically arise in utilizing CO.sub.2 particle blasting to remove contaminates or coatings from a surface include attaining complete coating or contaminate removal from the surface, attaining an acceptable removal rate, and preventing damage to the underlying surface or substrate. This is particularly a problem when carbon dioxide blasting is used to remove surface coatings from aircrafts wherein the substrate thickness is as low as 0.020 inches. In such an application, the carbon dioxide blasting process must be sufficient to remove the surface coating without damaging the thin substrate by creating stress buildup therein or work hardening the surface.
Several prior art methods and apparatuses are known for CO.sub.2 particle blasting. Some of these are set forth in U.S. Pat. Nos. 4,947,592, 5,018,667 and 5,050,805, and co-pending U.S. patent application 07/781,326 filed on Oct. 22, 1991, now U.S. Pat. No. 5,188,151, all of which are incorporated herein by reference. The process typically includes the formation of carbon dioxide pellets by producing carbon dioxide snow which is formed into carbon dioxide pellets by forcing the snow through circular die openings. As disclosed by U.S. Pat. Nos. 4,947,592 and 5,018,667, one method of forcing the CO.sub.2 snow through die openings is by use of a piston. Other apparatuses for forming CO.sub.2 pellets include rotary pelletizers.
The diameters of the prior art pellets, as dictated by the diameter of the die opening, are 0.120 inches and larger. After producing the pellets, they are then transported by a transport gas to an application nozzle designed to accelerate the transport gas flow and the entrained pellets to a high velocity. This exiting flow is directed at the work piece.
While the prior art pellets produced an acceptable removal of coatings or contaminates on a substrate, the pellets' size and speed created deleterious effects on the substrate itself. A cryogenic flow of pellets sufficient to produce complete coating or contamination removal at an acceptable rate can damage the substrate by deforming it and creating stresses.
Thus, there is a need in the art for a method and apparatus which provides sufficient coating or contaminate removal at an acceptable removal rate, which does not damage the substrate.